Tensiometer control system



Feb. 17, 1942.

G. P. LESSMANN TENSIOMETER CONTROL SYSTEM Filed March 21, 1939 2 Sheets-Sheet 2 INVENTOR ITNESSES N 5?' Patented Feb. 17, 1942 'rENsroMETER CONTROL SYSTEM Gerhard P. Lessmann, Forest Hills, Pa., assignor to Westinghouse Electric &' Manufacturing Company, East Pittsburgh, Pa., a corporation vof Pennsylvania Application March 2 1, 1939, Serial No. 263,146

13 Claims.

My invention relates, generally, to tensiometer control systems, and more particularly, to systems for controlling the tension on a strip of material as'it is passed through a strip rolling mill.

In the operation 'of steel strip rolling mills it has been found desirable to subject the strip to both front and back tensions at the roll stands. This .tensioning 'of the strip as it is being rolled I provides a greater reduction of the strip at each roll stand and thus requires fewer roll stands or reducing passes for a given strip reduction. Rolling the strip under tension also imparts certain desirable characteristics to the strip.

In rollingA steel strip under tension, it has been found that the amount f reduction of the strip for a given strip hardness and a given roll setting is a function of the amount oi tension to which the strip is subjected. It follows therefore that a strip of varying thickness and hardness may be reduced to a uniform thickness with a given roll setting if the tension is varied in accordance with variations in thickness and hardness of the strip to thus vary the reducingforces in accordance with variations in the resistanceto-reduc tion of the strip.

An object -of my invention is to provide a tensiometer control system which shall function to subject a strip of 'material, as it passes through a strip rolling mill, to such tensions as will enable the mill to produce strip of uniform thickness.

Another object Iojf my invention is to provide a strip tension control system for a strip rolling mill which shall. function to vary the strip tenn sion in accordance with variations in the resistance-to-reduction characteristics of the strip.

Another object of the invention is to provide a tensioning system for a tandem strip rolling mill which shall function to 'subject a strip to tension as it extends between adjacent roll stands of the mill, vary the strip tension in accordance with variations in the amount of strip between the roll stands, automatically maintain a predetermined amount of. strip between the roll stands and vary the amount of strip between the roll stands in accordance with variations in thickness of the strip.

A further object of the invention is to provide a tensioning system fora strip rolling mill which shall function to subject a strip to tensionY as it extends. between va Holl stand and another work device, vary the strip tension in accordance with variations in the amount of strip between kthe roll stand and the work device, automatically maintain a predetermined amount of strip be- (Cl. zzo-35) A control system for a strip rolling mill embodying All tween the roll stand and the work device, vary the amount of strip between the roll stand andy the work device in accordance with variations in the strip thickness and vary the load on the roll stand in accordance with .variations in the strip thickness.

Another object of the invention is to provide a tensioning system for a strip rolling mill which shall be simple in construction, simple and enioient in operation, and inexpensive to manufacture, install and maintain.

These and other objects and advantages of the A invention will be apparent from the following de- ,4 tailed description taken in connection with the accompanying drawings in which:

Figure 1 is a diagrammatic View of a tension the principal features of my invention; and,

Fig. 2v is a diagrammatic view of another embodiment of my invention. l

In carrying out my invention, in the embodiment of Figure 1, I provide a tensioning device Ill disposed to bear upon the strip l2 and subject it to tension by deecting it from its normal pass line between the preceding and succeeding roll stands M and it, respectively. The tensioning device i0 has a rising tension-deflection characteristic, that is, it subjects the strip to tensions which are inverse functions of the strip deflection.

A regulator i8 functions in response to move ments of the tensioning device it to control the speed of the drive motor 2@ of the roll stand i5 to vmaintain the strip deflection within a predetermined range. A' strip thickness responsive device 22 functions to adjust the regulator I8 to vary the limits of variations of .strip tension in accordance with variations in strip thickness.

VThe embodiment of the invention of Fig. 2 is similar to that of Fig. 1 with the exception that a current regulator 24 is embodied therein for controlling the loading' of the roll motor 20, and the setting 'ofV th current regulator is made responsive to the stripthickness.

Referring again toFig. 1 for a more detailed description of theprincipal features of the invention, the .tensioning device I0 is constructed and functions in substantially the same manner as the tensioning device of my copending application Serial No. 218,316,'ledf`July 9, 1938, now Patent No. 2,215,329, to which reference is made for a detailed description of its operation. The tensioning device i0 comprises, generally, a tensioning roller 26 mounted on an arm 28 of a crank 29 having a pivotal mounting 38. The roller 26 is disposed toengagethe strip I2 and subject it to tension by defiecting it from its normal pass line between-the roll stands I4 and I6 by a. iiuid pressure operated cylinder and piston 32 which acts upon an 'arm 34 of the crank 29.

An automatic pressure regulating valve 36 functions to maintain 'the desired operating pressure on the piston within the cylinder 32 and a cam 38 mounted for operation by the crank 29 operates through the stem 48 to adjust the valve 36 so that the tensioning device will have an increasing tension deiiection characteristic. That is, the tension on the strip I2l will increase with decreased deiiection of the strip and decrease with increased deection of the strip. The motor 42 is provided for selectively setting the cam 38 with respect to the operating crank 29 to provide different ranges of tension variation for any given vrange of strip deflection. This general operation of the tension device I8 is fully set forth in'my patent referred to hereinbefore.

The regulator I8 `functions to vary the current in the field Winding 44 of the drive motor `28 forthe roll stand I6. The regulator I8 coman arm 56 which is actuated against the bias of a spring 51 by a solenoid 58.

A circuit is provided for intermittently increasing and decreasing the energization o f the solenoid 58 to' cause its armaturey to reciprocate the arm 56. The energizing circuit for the winding of the solenoid 58 extends from one side of a source of current. indicated by conductor A through conductors 68, 62 and 64, the winding of the solenoid 58, conductor 86, resistor 68 and conductor 18 tothe other side of the source of current indicated by the conductor' B. A shunt# ing circuit is provided for Athe resistor 68 which of the drive motor 28 through a circuit which comprises the contact element 12 of a relay 14 and conductors 16 and 18. An energizing circuit is provided for the Winding of the relay 14 which extends from the conductor Athrough the conductors 68 and 62, the contact element 88V of the solenoid 58, conductor 82, the winding of the. relay 14 and conductors 18 and 18 to the conductor B.

It will be seen that this interconnection of the solenoid 58 and the relays 14 will cause intermittent increases and decreases of energization of the solenoid 58. In the position in which the elements of these circuits are shown, the winding of the solenoid 58will have full energization and solenoid 58 will actuate its armature to the -right moving the contact element 88 to closed circuit position. With the contact element 88 in the closed circuit position, the energizing circuit of the winding of the relay 14 hereinbeforev ment 88 to be moved to the open circuit position, deenergizing the winding of the relay 14 and causing the contact element 12 to be moved to the closed circuit position, thus again starting the cycle of intermittent increased and decreased energization of the Winding of the solenoid 58.

The intermittent engagement of the contact element 52 with the contact element 46 caused by the reciprocation of the 'armature of the solenoid 58 will cause intermittent energization of the winding of a relay 84 through a circuit which extends from the conductor A through the conductor 68, the arm53, contact elements 52 and 46, conductor 86, the winding of the relay 84, conductors 88., 38, 92 and 18 to the conductor B. In a similar manner, the intermittent engagement of the contact element 52 with the contact element 48 wilicause intermittent energization of a relay 94 through a circuit which extends from the conductor A through the conductor 68, the arm 53, contact elements 52 and 48, conductor 96, the Winding of relay 94 and conductors 98, 92and 18 to the conductor B.

'I'he intermittent energization of the relay 84 will cause intermittent movement of its contact element 98 to` closed circuit position in a shunting circuit for a resistor |88, the shunting circuit comprising a conductor |82 connected to one side of the resistor |88, the contact element 98 and conductors |84 and |86 to -the other side of the resistor |88. Similarly, the intermittent energization of the relay 94 will cause intermit- -tent movement of its contact element |88 from Aclosed circuit position in a shunting circuit for a resistor ||8, the shunting circuit comprising the conductors |86 and |84,v the contact element |88 and the conductor II2 connected to the resistor II8.

The resistors |88 and I|8 are connected in series circuit relation with the field winding 44 extends from the conductor B, through conductors 18 and 92, the resistors II8 and |88, conductor .II4, variable resistor II6, conductor I I8, the eld winding 44, conductor |28 4and conductor 68 to the conductor A.

In the operation of the regulator I8, it will be understood that the contact element 52 will alternately engage the contact elements 46 and 48 rto-intermittently energize the relays 84 and 94 to thus intermittently increase and decrease the current iiow in the iield winding 44 of motor 28. If, because of increased deection of the strip l2 by the tensioning device |8, the contact elements 46 and 48 are shifted upward so that in the alternate engagement of the contact element 52 with the contact elements 48 and 48 the contact element 62 will remain in engagement with the contact element 48 longer than Athe contactv element 46, this will result in a longer net energization of the relay 84 than the relay-84, thus producing a net decrease in the energization of the field winding and thus a net/decrease in the excitation of the motor 28. 'I'his decrease in the excitation of the motor 28 vwill tend to increase the speed of the motor 28 and the speed of the rolls of the roll stand I6 to take up the slack in the strip I2 and decrease the deection of the strip. When the deflection of the strip has been decreased, the contact elements 46 and 48 will again assume-a normal position with respect to the moving contact element 52 and such a net energization of the field winding 44 will be established as will maintain this normal deilection of the strip I2.

Since the tension applied to the strip |2 by the tensioning device I is a function of the deflection of the strip I2, automatic variation of the tension on the strip may be obtained by adjusting the regulator I8 to vary the deflection of the strip and to maintain a different degree of deflection of the strip. It will be understood that since the amount of reduction of the strip is a function of the tension applied to it, the amount of reduction of the strip may Vbe decreased by decreasing this tension, and provision is made whereby the thickness responsive device will control the tension applied to the strip, de-lv creasing the tension when the thickness is decreased, and increasing the tension when the thickness is increased, thus automatically providing the proper tension to produce a strip of substantially uniform thickness.

The thickness responsive device contact rollers |22 and |24 mounted for engagement on opposite sides of the strip and connected in a `Wheatstone bridge circuit |26 in such a. manner as to vary the unbalance of the bridge |26 an amount depending upon the resistance between the rollers |22 and |24 which will vary with the thickness of the strip I2, A

solenoid |28 is connected in circuit with thel bridge |26 soas to be responsive to the degree of unbalance of the bridge |26. The solenoid |28 actuates the arm |30 of a bell crank |32 having a fixed pivotal mounting |34 and another arm |36, which supports the pivotalmountlng 55 for the bell crank 54 hereinbefore described; The solenoid |28 actuates its armature against the biasing tension of a spring |35. Thus it will be seen that the position of the pivotal mounting 55 will be determined by the thickness of the strip I2, and the connections of the, solenoid |28 with the Wheatstone bridge |26 are such` as vto raise the pivotal mounting 55 when the thickness of the strip decreases, and to lower it when the strip thickness increases. Raising lof the pivotal mounting 55 will raise-the range of movement of the movable contact element 52, and since the range of movement of the contact element 52 determines the position at which the regulator I8 maintains the deflection of the strip I2, the'decreased thickness of the strip'as measured by the thickness responsive de- -vice 22 will cause such a readjustment of the regulator I6 as will increase the deflection of the strip l2v and maintain this increased deflection and thus decrease the tension maintained on the strip I2.

In a similar manner, increases in the thicknessof the strip l2, as measured by the thickj ness responsive device 22, will cause the range of deflection of the movable contact element 52 to be lowered, thus decreasing the deflection of the strip` I2 and increasing the tension applied to the strip I2.

Thus it will be apparent that, inthe operation of the system of Fig. 1, the strip I2 will be subjected to a tension by the tension device I0 as it passes between theroll stands |4 and I6, which will vary inversely with the deflection of the strip |2, and the amount of tension on the strip will be governed by the thickness of the strip as determined by the thickness responsive device 22, which acts to so adjust the regulator I8 as to cause the regulator I8 to control the speed of the drive motor`20 and thereforethe 22l comprisesv roll stand I6, to maintain a strip deflection which will produce the necessary strip tension corresponding to the strip thickness.

The embodiment of the invention shown in Fig. 2 is generally similar to that of Fig. 1 with the exception that the loading of the drive motor 20 is also controlled by the strip thickness responsive device 22. Like reference characters are applied to the corresponding elements of the -syste'ms of Figs. 1 and 2.

motor will have to be varied in accordance with variations in strip tension if the rolling load is to be maintained constant. The load on the motor 20 is controlled by a constant current regulator 24. which comprises a solenoid |40 whose armature acts against the bias of a spring |42, tending to move a contact element |44 to closed circuit position in a shunting circuit for a resistor |46 whichis connected in series circuit relation with the field winding 44 of the drive motor 20. The circuit for the field winding 44 extends from the conductor through conductors 60 and |48, the variable resistor H6, conductor |50, the field Winding 44, conductor |52, resistors |00 and ||0, conductor ||`2, resistor |46 and con- A ductor |54 to the conductor B. The shunting circuit for the resistor |46 controlled by the movable contact element |44 of the solenoid |40 comprises the conductors I|2 and |56, the contact element |44 and the conductor |54.

Normally, the constant current regulator 24 will be energized in accordance with the armature current of the motor 20 and when this current increases beyond a predetermined amount the contact element |44 will close the shunting circuit for the resistor |46, thus increasing the excitation of the motor 20, causing it to tend to decrease its speed and thus decreasing its armature current. Inthis manner the constant current regulator would normally tendto maintain a constant' load on the motor' 20. The'conl stant current regulator 24, however, is modified to cause its effective current setting to Vary in accordance with the thickness of the strip I2 to thus vary the total load on the motor 20 in accordance with variations in tension applied to 4strip I2.

A circuit is provided to render the coil of the solenoid |40 responsive to the current of the motor 20 and for varying the sensitivity of the current regulator in accordance with the thickness of the strip. This circuit is influenced by the potential drop across a series winding |58 in the armature circuit of the motor 20, which may be either a series field or an interpole winding, and extends from one side of the winding I 58 through conductor |60, the coil of the solenoid |40, conductor |62, resistors |64 and |66, conductor |68, variable resistor |10 and conductor '|12 connected tothe other side of the series winding |58.

The energizmation of the winding of the solenoid |40 is innuenced by the intermittent connection of the resistors |64 and |66 in its circuit in a manner similar to the intermittent connection of the resistors |00 and ||0 in the circuit of the before described in connection with Fig. 1. The control of the resistors |64 and |66 comprises movable contact elements |14 and |16 mountedv upon the arm 53h of the bell crank 54 and disposed to alternately engage nxed contact elements 11s and lso, respectively. The intermittent engagement of the contact elements |14 and |18 will'cause intermittent energization of a relay |82 through a circuit which extends from the conductor A through the conductor 60, the arm 53, contact elements |14 and |18,conductor |84, the coil4 of the relay |82, and conductors |86 and to the conductor B. Similarly, relay |68 is disposed to be intermittently energized by the intermittent engagement of the contact elements |16 and |80 through a circuit which extends from the conductor A through conductor 60, the arm 53, contact elements |16 and '180, conductor |90, the coil of the relay |88 and conductors |92, |86

and 10 to the conductor B. a

The intermittent energization of the relay-|82 will cause its movable contact element |94 to intermittently close a shunting circuit for the resistor |66 which extends from one sidev of the resistor |66 through the conductor |96, the contact element |94, and the conductors .|98 and 200 to the other side of the resistor |66. Similarly, intermittent energization of the relay |88 will cause intermittent movement of its movable contact element 202 away from closed circuit position in a shunting circuit for the resistor |64, the shunting circuit extending from one side of the resistor |64 through the conductors 200 and |98, the contact element 202 and the conductor 204 to the other side of the resistor |64.

The movable contact elements |14 and |16 are mounted upon leaf spring members 206 and 208,

respectively, so that as the range of movement of the arm 53 is varied by the thickness responsive device 22 as hereinbefore described, the relative periods of engagement of the contact element |14 with the contact element |18 with respect to the engagement of the contact element strip, the pivotal mounting 55 will move upward and therefore the range of movement of the arm 53 will be moved upward. This upward movement of the range of movement of the arm 53 will cause the contact element |14 to engage the contact element |18 longer than the contact element |16 is in engagement'with the contact element |80 on each reciprocation of the arm 53. Thus the relative periods of energization of relays |82 and |88 will cause a net decrease in the resistance of the circuit of the winding of the solenoid |40, thus permitting a smaller motor load current to actuate the armature of the solenoid |40 and resulting in a smaller load current on the motor 20.

Similarly, increasing thickness of the strip will "require that a higher tension be applied to the' strip, and this higher tension will be applied der control of the strip thickness responsive deliieldy winding '44 by the regulator I8 as hereinvice 22 and the extra load current on the motor 20 to supply this extra tensioning load will be automatically produced by the adjustment oi the current regulator 24 by the thickness responsive device 22.

It isto be understood that the systems of Figs. 1 and 2 may be applied to either two roll stands as shown and described herein, or one roll stand and either-a winding or unwinding reel of a strip mill, and that the systems may be applied to all or the pairs of stands of a tandem mill in which more than a single pair oi roll stands is employed. It is to be understood that any suitable tensioning device may be employed for automatically tensioning the strip I2 so long as it has a rising tension-deflection characteristic as herein described. Also, any suitable current regulator which will function in substantially the manner herein described or any suitable thickness responsive device may be employed in place of those described herein. e

It will be seen that I'haveprovided strip ten`v cation by one skilled in the art, the embodiments herein described being merely illustrative of the principles of my invention.

I claim as my invention.

1. In a system for controlling the tension ona strip of material as it passes between two adjacent Work devices, force operated tensioning means'for applying such transverse forces to the strip as will cause the strip tension to vary as a function of the amount of deection of the strip from its normal pass line between the work devices, regulating means responsive to the degreevof deflection of the strip for maintaining' the strip deiiection within a predeterminedA range, and continuously operating means responsive to the strip thickness for automatically causing said regulating means to vary the range of deflection of the strip in accordance with variations of the strip thickness.

2. In a system for controlling the tension on a length of material, a pair of material reducing units between which the length of material extends as it is being reduced by said reducing units, a bearing member engaging the length of material to deflect it from a normal pass line and thus subject`the length of material to tension, force applying means for causing said bear` ing member to subject the length of material to tensions which are functions of the amount of deection of the length of. material, regulator means foi` maintaining the deflection of the length of material within a predetermined range,

and continuously operating means responsive to resistance-to-rolling Acharacteristics of the length of material for automatically causing said regulator means to vary the limits of the range oi' deflection of the length of material.

3. In a system for controlling the tensionon a length of material as it extends and passes between a pair of reducing units which operate on the material toreduce its sectional area, a bear- 'characteristics of the strip.

ing-member engaging the length of material to deflect it from a normal pass line and thus subject the length of material to tension, force applying means for causing said bearing member to subject the length of material to tensions which are functions ofA the amount of deflection of the length of material, means including regulator means actuated in accordance with the deflection of the length of materialv for maintaining the deflection of the length of material within a predetermined range, and continuously operating means responsive to resistance-to-rolling characteristics of the length of material for automatically adjusting the setting of said regulator means to thereby vary the limits of the range of deflection of the length of material to' vary the tensions'applied thereto in accordance with variations of said characteristics.

4. In a strip tension control system for a tandem strip rolling mill, a bearing member engagin'g the strip as it extends between adjacent roll stands of the mill to deflect the strip from a `normal pass line and thus subject the strip to tension, means applying such forces to said bearing member as to produce strip tensions which vary as an inverse function of the strip deflection, regulator means responsive to the strip deflection` and controlling the relative speeds of the rolls of the adjacent roll stands to automatically maintain the strip deflection within a predetermined range, and continuously operating'r means responsive to resistance-to-rolling characteristics of the strip for automatically adjusting saidregulator means to thereby vary the limits of the range of deflection of the strip thereby varying the strip tension in accordance with variations in said resistance-to-rolling 5. In a strip tension controlsystem for a tandem strip rolling mill, a bearing member engaging the strip as it extends between adjacent roll stands of the mill to subject the strip to tension by deflecting the strip from a normal pass line, actuating means for applying such forces to said bearing member as will produce tensionsin the v striphich vary as an inverse function of the strip deflection, regulator means responsive to the strip ldeflect-ion and controlling the relative speeds of the rolls of the adjacent roll stands to maintain the strip deflection within a predetermined range, and means responsive to the resistance-to-rolling characteristics of the strip for adjusting said regulator means to vary the limits of the range of deflection of the strip,

thereby varying the strip tension in accordance `with variations of the resistance-to-rolling characteristics of the strip.

6. In a system for controlling the tension on a strip of material as it passes between' adjacent roll stands of a strip rolling mill, tensining means for applying such transverse forces to `the strip as will cause the tension on the strip to vary as an inverse function of the amount of deflection of the strip'from a normal position, means responsive to the degree of deection of the strip for maintaining the deflection within' a predetermined range, means responsive to the 4- resistance-to-reduction of the strip for varying the limits of the range of deflection of the strip, regulating means for maintaining an adjustable substantially constant -pwer input to the succeeding one of the roll stands, and means wherebythe value of the power input to the succeeding roll standsl is adjusted by said means responsive to the resistance-to-reduction of the strip.

7. In a strip rolling mill having adjacent sets of electric motor driven rolls between which the strip extends as it passes through the mill, a bearing member engaging the strip between the sets of rolls to deflect the strip from a normal pass line and thus subject the strip to tension, means applying such forces to said bearing member as to produce. strip tensions which vary as an inverse function of the strip deflection, regulator means responsiveto the strip deflection and controlling the relative speeds of the driving motors of the adjacent sets of rolls to maintain the strip deflection within a predetermined range, gauge means responsive to the thickness of the strip for adjusting said regulator means to vary the limits of the range of deflection of the strip, means for maintaining a predetermined load on the driving motor of the succeeding set of rolls, and means whereby said gauge means adjusts the predetermined load value in accordance with variations in the thickness of the strip.

8. In a strip rolling mill having a pair of adjacent sets of electric motor driven reducing rolls between which the strip extends as it passes through the mill, a bearing member engaging thev strip between the sets of rolls to deflect the strip from a normal pass line and thus subject the strip'to tension, means applying such forces to said bearing member as to produce strip tensions which vary as an inverse function of the strip deflection, regulator means responsive to the strip deflection and controlling the speed of the succeeding roll motor to maintain .the strip deflection within a predetermined range, gauge means responsive to the thickness of the strip for adjusting said regulator means to vary the limits of the range of deflection of-'the strip, current regulator means forrmaintaining a predetermined load on the succeeding roll motor, and means whereby said gauge means adjusts the predetermined load value 'in accordance with variations in the strip thickness.

' 9. In a tension control system for a strip rolling mill, means subjecting the strip to tension as it passes between adjacent roll stands, control means for automatically maintaining a substantially constant load on the succeeding one of a pair of adjacent sets of rolls, regulator means, and continuously operating means responsive to the resistance-to-reduction characteristics of the strip for automatically causing said regulator means to vary the tension on the strip and the load on the succeeeding set of rolls in accordance with said resistance-to-reduction.

10. In a tension control system for a strip rolling mill, means subjecting the strip to tension as it passes between adjacent roll stands, load regulator means for automatically maintaining a substantially constant load on the succeeding one of a pair ofadjacent sets of rolls, and continuously operating -means responsive to the thickness of the strip for automatically varying the tension on the strip and causing the regulator mean to vary the load on the succeeding setv of rolls accordance with said thickness. 11,j In a tension control system for a strip rolling mill, means subjecting the strip to tension as it passes between adjacent roll stands, load regulator means for automatically maintaining a substantially constant load on the succeeding one of 'a pair of adjacent sets of rolls, and continuously operating means responsive to the thickness of the strip for automatically causing Said "regulator means to vary the 10aa 0n Said Suc' said i the strip to tensions which are an inverse function of the deection of the strip, regulator means l0 responsive to the deflection of the strip and controlling theexcitation of the electric drive motor for the succeeding set of rolls to maintain the strip deection within a predetermined range,

and means responsive to the strip thickness for'i adjusting said regulator means to vary the limits of the range of deection of the strip in accord- `ance with variations in the thickness of the strip.

13. In a control system for a tandem strip rolling mill having an electric drive motorffcr the 20 succeeding one of a pair of adjacent sets of reducing rolls, tensioning means acting to deflect the strip from a normal pass line and thus subject the strip to tension and operative to subject the strip to tensions which are an inverse function of the deflection of the strip, regulator means vresponsive to the deflection of the strip and controlling the excitation of the electric drive motor for the succeeding set of rolls to maintain the strip deflection within a predetermined range, means responsive to the strip thickness for adjusting said regulator means to vary the limits ofthe range of deflection of the strip in accordance with variations in the thickness of the strip, current regulator means responsive to the load vcurrent of the drive motor for the succeeding set of rolls-and operating tovary the excitation of said motor to maintain a substantially constant predetermined load on said motor, and means including said thickness responsive means for varying the value of the predetermined load current of said motor in accordance with variations in the strip thickness.

GERHARD P. LESSMANN. 

